MINOR: quic: implement send-retry quic-initial rules

Define a new quic-initial "send-retry" rule. This allows to force the
emission of a Retry packet on an initial without token instead of
instantiating a new QUIC connection.

include/haproxy/quic_sock-t.h

@@ -27,6 +27,7 @@ struct quic_receiver_buf {
 };
 
 #define QUIC_DGRAM_FL_REJECT			0x00000001
+#define QUIC_DGRAM_FL_SEND_RETRY		0x00000002
 
 /* QUIC datagram */
 struct quic_dgram {